Okay, so there's no letter U in the Greek alphabet. This immediately presents a problem. However, when directly translating names from other languages to Greek, the letter U is replaced by the letter V.
But, modern numerals as we know them did not exist. Roman numerals did, however exist. The number 2 in Roman Numerals is II.
Thus, U2 would've been VII, which is of course, 7. So the band would've been called 7. Maybe that's why Edge kept wearing those #7 shirts